An excellent cryogenic magnetic cooler: magnetic and magnetocaloric study of an inorganic frame material

Abstract

The cryogenic magnetic coolers based on the magnetocaloric effect (MCE) of paramagnetic compounds usually need relatively large fields to realise a practical cooling performance. Thus, it is of significance to search for molecule-based cryogenic magnetic coolers with high performance of the MCE under low fields (smaller than 2 T), considering the ready availability of low fields by commercial Nd–Fe–B permanent magnets. In this work, the crystal structure, magnetic susceptibility and isothermal magnetization for the inorganic compound Gd(OH)SO₄ (1) have been investigated. The title compound exhibits a 3D structure with Gd-oxygen chains as the supramolecular building units. Magnetic characterisations indicated that adjacent Gd^III ions show weak magnetic couplings in 1. Because of the integration of large metal/ligand mass ratio, weak magnetic couplings and a dense framework, the observed maximum entropy change (−ΔS^max_m) for 1 was up to 53.5 J kg⁻¹ K⁻¹ or 276 mJ cm⁻³ K⁻¹ for ΔH = 7 T and T = 2 K, comparable to the performance of commercial gadolinium gallium garnet Gd₃Ga₅O₁₂ (GGG, −ΔS^max_m = 38.4 J kg⁻¹ K⁻¹ or 272 mJ cm⁻³ K⁻¹ with ΔH = 7 T). Notably, the −ΔS^max_m of 1 still reaches 27.5 J kg⁻¹ K⁻¹ at T = 2 K and ΔH = 2 T, and 38.3 J kg⁻¹ K⁻¹ at T = 2 K and ΔH = 3 T, which already surpasses GGG (about 24 J kg⁻¹ K⁻¹ with ΔH = 3 T) and makes 1 a superior cryogenic magnetic cooler for low-temperature applications, even in low fields.

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Introduction

The changes of the isothermal magnetic entropy and adiabatic temperature of a solid material induced by changes in the applied magnetic field are defined as the magnetocaloric effect (MCE), which was first discovered by Warburg in metallic iron in 1881 and can be used to attain ultra-low temperature by adiabatic demagnetization.¹ Cryogenic magnetic cooling based on the MCE has been considered as an environmentally friendly and energy-saving technique.² Recently, molecular cryogenic magnetic coolers have caught the eye of researchers due to their intrinsic characteristics such as stoichiometric composition, monodispersity, modifiability, etc.³

Four effective strategies have been proposed by Tong and coworkers to design and synthesize optimized molecule-based cryogenic magnetic coolers, including boosting ground-state spin, lowering magnetic anisotropy, weakening magnetic interactions, and reducing the molecular weight.^3a Thus, the sound assembly of paramagnetic metal ions and bridging ligands directed by the previous lessons and strategies play a vital role in the cooling performance of the resultant compounds.³ Hitherto, most of the explored molecule-based cryogenic magnetic coolers are Gd-based clusters and coordination polymers (CPs),³ which is mainly ascribable to the characteristics of the Gd^III ion (large spin ground state, magnetic isotropy and weak exchange coupling).^4,5 Among the various organic ligands used, carboxylate ligands are broadly studied.^3,6 Coordination-driven assembly of the Gd^III ion and light carboxylate ligands has given birth to a series of impressive Gd-based magneto-coolants, examplified by 3D [Gd(HCOO)₃] (−ΔS^max_m = 55.9 J kg⁻¹ K⁻¹ or 216 mJ cm⁻³ K⁻¹ with ΔH = 7 T),^7a 2D [Gd₂(C₂O₄)₃(H₂O)₆·0.6H₂O] (−ΔS^max_m = 46.6 J kg⁻¹ K⁻¹ or 118 mJ cm⁻³ K⁻¹ with ΔH = 7 T),^7b 2D [Gd(C₂O₄)(H₂O)₃Cl] (−ΔS^max_m = 48.0 J kg⁻¹ K⁻¹ or 144 mJ cm⁻³ K⁻¹ with ΔH = 7 T),^7c and 1D [Gd(HCOO)(OAc)₂(H₂O)₂] (−ΔS^max_m = 45.9 J kg⁻¹ K⁻¹ or 110 mJ cm⁻³ K⁻¹ with ΔH = 7 T).^7d

Paralleling the extensively studied organic ligands, the light inorganic ligands are also promising candidates for constructing Gd-based magnetic cryocoolants bearing large MCE in gravimetric units (J kg⁻¹ K⁻¹) and volumetric units (mJ cm⁻³ K⁻¹) thanks to the weak exchange interactions and dense frame in the resulting compounds.^8,9 The shielded 4f orbitals of Gd^III make the Gd^III–Gd^III magnetic coupling usually weak, even if the ligand is a bridging-oxygen atom, which is well corroborated in Gd(OH)₃ (−ΔS^max_m = 62.0 kg⁻¹ K⁻¹ or 346 mJ cm⁻³ K⁻¹ with ΔH = 7 T) and Gd₂O(OH)₄(H₂O)₂ (−ΔS^max_m = 59.1 J kg⁻¹ K⁻¹ or 217 mJ cm⁻³ K⁻¹ with ΔH = 7 T).^8a Furthermore, the inorganic Gd-based magnetic coolants usually display good chemical and thermal stability.^8b,c The effectiveness and efficiency of inorganic ligands to fabricate ideal Gd-based cryogenic magnetic coolers has been well testified in publications.^8,9 Representative examples include the early investigated Gd₂(SO₄)₃·8H₂O,^9a,b recently reported GdF₃ (−ΔS^max_m = 71.6 J kg⁻¹ K⁻¹ or 506 mJ cm⁻³ K⁻¹ with ΔH = 7 T),^9c GdPO₄ (−ΔS^max_m = 62.0 J kg⁻¹ K⁻¹ or 376 mJ cm⁻³ K⁻¹ with ΔH = 7 T),^9d and K₃Li₃Gd₇(BO₃)₉ (−ΔS^max_m = 56.6 J kg⁻¹ K⁻¹ or 278 mJ cm⁻³ K⁻¹ with ΔH = 7 T).^9e Different from the Gd-based molecular magnetic coolers supported by organic ligands, their counterparts like Gd₂(SO₄)₃·8H₂O, Gd(OH)₃, GdF₃ and GdPO₄ have usually been known for many years, which gives us a hint that utilising known results is also an avenue to explore brilliant molecular cryogenic magnetic coolers under the guidance of molecular design strategies.

Herein, we report the structure, magnetic susceptibility and isothermal magnetization of an inorganic frame material with the formula of Gd(OH)SO₄. This material features weak magnetic exchange, low molecular weight (270.32 g mol⁻¹) and dense frame structure (5.163 g cm⁻³), which is helpful for boosting the maximum entropy change (−ΔS^max_m). The −ΔS^max_m derived from isothermal magnetization is up to 53.5 J kg⁻¹ K⁻¹ or 276 mJ cm⁻³ K⁻¹, comparable to the performance of commercial gadolinium gallium garnet Gd₃Ga₅O₁₂ (GGG, −ΔS^max_m = 38.4 J kg⁻¹ K⁻¹ or 272 mJ cm⁻³ K⁻¹ with ΔH = 7 T) and its derivative Gd₃(Ga_1−xFe_x)₅O₁₂ (GGIG).¹⁰ It is notable that the −ΔS^max_m of 1 still reaches 27.5 J kg⁻¹ K⁻¹ (T = 2 K and ΔH = 2 T) and 38.3 J kg⁻¹ K⁻¹ (T = 2 K and ΔH = 3 T) even in low fields, which already surpasses GGG (about 24 J kg⁻¹ K⁻¹ with ΔH = 3 T). All these magnetic characteristics make 1 an outstanding cryogenic magnetic cooler.

Experimental

Materials and instrumentation

All chemicals were analytical grade and used directly. IR data were collected on a MAGNA-560 (Nicolet) FT-IR spectrometer with KBr as pellets. Experimental powder X-ray diffraction (PXRD) data were collected on a Bruker D8 FOCUS diffractometer with a Cu-target tube and a graphite monochromator. Simulated PXRD data were obtained from the single crystal X-ray diffraction (SCXRD) data via the Mercury program. Thermogravimetric (TG) data were collected on a Rigaku Thermo plus EVO2 TG-DTA8121 analyzer. The magnetic data were collected using a MPMS XL-5 SQUID magnetometer. The diamagnetic corrections were finished with Pascal's constants.

Synthesis of Gd(OH)SO₄ (1). 1 could be prepared according to the literature procedure.¹¹1 could also be synthesized by the following method. A mixture of Gd₂(SO₄)₃·8H₂O (0.075 g, 0.10 mmol), H₂C₂O₄·2H₂O (0.189 g, 1.50 mmol), urea (0.015 g, 0.25 mmol) and H₂O (10 mL) was sealed in a Teflon-lined autoclave (20 mL) and heated to 180 °C for 7 days and then slowly cooled to ambient temperature. Yield: ca. 15% based on H₂C₂O₄·2H₂O. IR (cm⁻¹): 3480(s), 3425(s), 2970(m), 2917(w), 1621(s), 1382(w), 1172(s), 1087(s), 990(s), 867(s), 773(s), and 600(s).

Crystallographic data collection and refinement

The SCXRD data were collected on a XtaLAB-mini diffractometer at 293(2) K with Mo-Kα radiation (λ = 0.71073 Å) by ω scan mode. SHELX-2016 software was used to solve the frame of 1.¹² Detailed crystallographic data are presented in Table 1 and the selected bond lengths and angles are given in Table S1 (ESI†). Further details on the structural investigation is available from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49) 7247-808-666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository number CSD-261284 (Gd(OH)SO₄).

Table 1 Crystal data and structural refinement for Gd(OH)SO₄

Chemical formula	Gd(OH)SO4
Formula mass/g mol^−1	270.32
T/K	293(2)
λ/Å	0.71073
Crystal system	Monoclinic
Space group	P21/n
a/Å	4.3950(4)
b/Å	12.2030(8)
c/Å	6.7606(6) Å
β/°	106.434(9)
V/Å^3	347.77(5)
Z	4
D c/g cm^−3	5.163
μ/mm^−1	19.547
F(000)	484
Total reflns	1144
Unique reflns	612
R int	0.0254
Final R indices [I > 2σ(I)]	R 1 = 0.0308, wR2 = 0.0817
R indices (all data)	R 1 = 0.0320, wR2 = 0.0833
GOF on F^2	1.054

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Results and discussion

Description of crystal structures

Because the structure of 1 has been reported,¹¹ the structural delineations are introduced briefly. SCXRD analysis indicates that compound 1 crystallizes in the monoclinic space group of P2₁/n. The asymmetric unit of 1 has one crystallographically independent Gd atom, one sulfate (SO₄²⁻), and one hydroxy anion (OH⁻). As displayed in Fig. 1a, all Gd atoms are in the [GdO₉] capped square antiprism geometry. The SO₄²⁻ groups have the Harris notation of [6.2211] to link six Gd atoms,¹³ while the OH⁻ moieties exhibit μ₃ mode to link three Gd atoms. The presence of sulfate and OH⁻ moieties in the frame has also been confirmed by their corresponding characteristic IR peaks (3480, 867, 773 cm⁻¹ for OH⁻ and 1172 cm⁻¹ for sulfate) (Fig. S1, ESI†). The μ₃-OH⁻ together with the η²-O-atom from sulfate connect symmetry-related Gd-ions to form the 1D Gd-oxygen chain (Fig. 1b), which is further connected to the other η²-O-atom of sulfate to generate the 2D layer (Fig. 1c). The adjacent 2D layers connect with each other via the η¹-O-atom of sulfate to give rise to the resultant 3D dense framework (Fig. 1d).

Fig. 1 (a) The coordination environments of sulfate and Gd atoms; (b) the 1D Gd-oxygen chain; (c) the 1D Gd-oxygen chain connected by sulfate to form the 2D layer; (d) the 2D layer linked by an additional O-atom of sulfate to generate the 3D structure.

TG analysis for 1 was measured under air atmosphere between 25 and 1000 °C (10 °C min⁻¹). As illustrated in Fig. S2 (ESI†), no apparent weight loss was observable from ambient temperature to 600 °C, implying good thermal stability of 1. An apparent weight loss is present upon further heating of the sample above 610 °C, indicating the collapse of the framework. To confirm the phase purities before property measurements, PXRD was conducted. Fig. S3 (ESI†) illustrates that the positions of the experimental peak values are in accordance with the fitted ones derived from SCXRD, suggesting the purity of the samples. The distinct intensities between the peaks may be attributable to the variation in the preferred orientation of the experimental sample.

Magnetic properties

The mole magnetic susceptibility (χ_M) of 1 was investigated in the solid state in the temperature range of 2–300 K at 1 kOe field. All magnetic measurements were obtained with crushed crystalline samples. The observed χ_MT product of 8.04 cm³ K mol⁻¹ at 300 K basically matches with the value for one isolated Gd^III (7.88 cm³ K mol⁻¹, ⁸S_7/2, g = 2) in 1 (Fig. 2a). During the process of cooling, the χ_MT products of 1 decline very slowly from 300 K to 10 K. Below 10 K, an apparent fall was present with the minimum value of 7.51 cm³ K mol⁻¹ at 2 K. The declining trend of the χ_MT vs. T plot for 1 manifests that adjacent Gd^III ions feature antiferromagnetic (AF) coupling, which is further corroborated by the negative Weiss constant θ = −0.22 K from Curie–Weiss simulation (Fig. 2a). The accordance between zero field cooled (ZFC) and field cooled (FC) magnetizations excluded the possibilities of long-range magnetic ordering (Fig. 2b).

Fig. 2 (a) The plot of χ_MT vs. T for 1 (red part for the Curie–Weiss simulation). (b) FC and ZFC magnetization of 1 in the dc field of 50 Oe. (c) Variable-field magnetization plots at specific temperatures for 1. (d) −ΔS_m derived from experimental magnetization data of 1 at distinct fields and temperatures.

The variable-field magnetization (M) characterizations were finished in the scope of 0–7 T at 2–10 K for 1 (Fig. 2c). The curves of M vs. H show a gradual increase with the increasing fields. The M is 7.41 Nβ at 2 K and 7 T, basically agreeing with the theoretical value of 7.0 Nβ for one Gd^III (g = 2, S = 7/2). The weak magnetic exchange interactions, together with high metal/ligand mass ratio, suggest that compound 1 holds great promise as a cryogenic magnetorefrigerant. As a vital factor in appraising the MCE, magnetic entropy change −ΔS_m can be inferred from experimental magnetization data via Maxwell equation .³ The magnetic fields and temperature-dependent entropy changes are presented in Fig. 2d, with the maximum −ΔS_m (−ΔS^max_m) up to 53.5 J kg⁻¹ K⁻¹ at 2 K with ΔH = 7 T. The experimental −ΔS^max_m is lower than the theoretical one of 64.0 J kg⁻¹ K⁻¹ for one free Gd^III in the chemical formula (judged by n_GdR ln(2S_Gd + 1)), which may be ascribable to the global AF behaviours in 1. Among reported Gd^III-based molecular magnetic cryocoolers, −ΔS^max_m above 40.0 J kg⁻¹ K⁻¹ is limited.^3,4a The −ΔS^max_m for 1 is 276 mJ cm⁻³ K⁻¹ from the volumetric aspect, which is still very competitive among the Gd^III-containing complexes.^3,4a Compared with the sulfate-based molecular cryogenic magnetic coolers (Table 2), the maximum MCE per unit mass of 1 (53.5 J kg⁻¹ K⁻¹) is slightly larger than those reported for (3,12)-connected [Gd₄] cluster-based compound [Gd₄(SO₄)₄(OH)₄(H₂O)₄] (51.3 J kg⁻¹ K⁻¹),^8b (3,11)-connected [Gd₄] cluster-based compound [Gd₄(SO₄)₃(OH)₄(C₂O₄)(H₂O)₅]·H₂O (51.5 J kg⁻¹ K⁻¹),^5e and 3D [Gd(HCOO)(SO₄)(H₂O)] (49.9 J kg⁻¹ K⁻¹)¹⁴ because of the synergetic effect of weak magnetic interactions and small molecular weight. However, due to the dense frame characteristic, the −ΔS^max_m per unit volume of 1 (276 mJ cm⁻³ K⁻¹) is obviously higher than the related ones with 199 mJ cm⁻³ K⁻¹ for the Gd-sulfate system,^8b 190 mJ cm⁻³ K⁻¹ for the Gd-sulfate-oxalate system,^5e and 190 mJ cm⁻³ K⁻¹ for the Gd-sulfate-formate system.¹³ It is, indeed, more significant for application when appraising the cryogenic magnetorefrigerants from the point of volumetric unit.¹⁵ Notably, the −ΔS^max_m of 1 is larger than most of the reported Gd^III-based molecular magnetic coolers fabricated by organic ligands (Table 2), implying the efficiency of light inorganic ligands in the synthesis of attractive molecular magneto-cryocoolants. The high −ΔS^max_m of 1 is mainly due to the weak magnetic coupling between adjacent Gd^III ions, small molecular weight and dense framework characteristics. Furthermore, the −ΔS_m per unit mass still reaches the values of 27.5 J kg⁻¹ K⁻¹ (T = 2 K and ΔH = 2 T) and 38.3 J kg⁻¹ K⁻¹ (T = 2 K and ΔH = 3 T) even in lower fields, which already surpasses the performances of commercial gadolinium gallium garnet Gd₃Ga₅O₁₂ (GGG) (about 24 J kg⁻¹ K⁻¹ with ΔH = 3 T)¹⁰ and makes 1 a promising cryocoolant for potential application, considering the ready availability of low fields by commercial Nd–Fe–B permanent magnets.^3f,17

Table 2 Comparison of −ΔS^max_m (larger than 40.0 J kg⁻¹ K⁻¹) between 1 and sulfate-based molecular cryogenic magnetic coolers together with some selected Gd-complexes fabricated by organic ligands

Complex	D	−ΔS^maxm
		[J kg^−1 K^−1] [mJ cm^−3 K^−1]
−ΔS^maxm [mJ cm^−3 K^−1] = −ΔS^maxm [J kg^−1 K^−1]*ρcald [g cm^−3] D: dimensionality.
[Gd(HCOO)3]^7a	3D	55.9	216
Gd(OH)SO4 (this work)	3D	53.5	276
[Gd4(SO4)3(OH)4(C2O4)(H2O)5]·H2O^5e	3D	51.5	190
[Gd4(SO4)4(OH)4(H2O)4]^8b	3D	51.3	199
[Gd(HCOO)(SO4)(H2O)]^14	3D	49.9	190
[Gd(C2O4)(H2O)3Cl]^7c	2D	48.0	144
[Gd(OAc)3(H2O)0.5]^16a	1D	47.7	106
[Gd(HCOO)(C8H4O4)]^16b	2D	47.0	125
[Gd2(C2O4)3(H2O)6·0.6H2O]^7b	2D	46.6	118
[Gd(HCOO)(OAc)2(H2O)2]^7d	1D	45.9	110
[Gd(OAc)3(MeOH)]^16a	1D	45.0	97
[Gd(C4O4)(C2O4)0.5(H2O)2]^16c	3D	44.0	128
[Gd(cit)(H2O)]^16d	2D	43.6	115
{[Gd(OAc)3(H2O)2]}2·4H2O^16e	0D	41.6	83
{[Gd2(IDA)3]·2H2O}^16f	3D	40.6	101

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Conclusions

The structure, magnetic susceptibility and isothermal magnetizations for the inorganic Gd-based coordination polymer (CP) Gd(OH)SO₄ (1) have been investigated. The title CP exhibits the 3D dense architecture with a Gd-oxygen chain as supramolecular building units. Magnetic characterisations indicated that adjacent Gd^III ions show weak magnetic exchange in 1. Because of the integration of the large isotropic spin of Gd^III, the dense structure and weak magnetic couplings, the observed maximum entropy change (−ΔS^max_m) for 1 was 53.5 J kg⁻¹ K⁻¹ for ΔH = 7 T and T = 2 K and 38.3 J kg⁻¹ K⁻¹ for T = 2 K and ΔH = 3 T, which already surpasses the commercial gadolinium gallium garnet Gd₃Ga₅O₁₂ (GGG) (38.4 J kg⁻¹ K⁻¹ with ΔH = 7 T and about 24 J kg⁻¹ K⁻¹ with ΔH = 3 T), indicating it to be an excellent cryogenic magnetic cooler. This work highlights the role of inorganic ligands in the generation of excellent molecular cryogenic magnetic coolers. Further work will focus on utilising other known results to explore brilliant molecular cryogenic magnetic coolers under the guidance of molecular design strategies.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of China (21571111, 21601099 and 21601100), the China Postdoctoral Science Foundation (2016M592137), the Young Scientist Foundation of Shandong Province of China (ZR2016BB25), and the project of applied and fundamental research of Qingdao City of China (16-5-1-87-jch).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional property characterization, and selected bond lengths and angles. See DOI: 10.1039/c8qm00439k

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